Sawtooth generator



July 17, 1956 G. H. NIBBE SAWTOOTH GENERATOR Filed March 26, 1946 PLATE VOLTAGE SUPPLY OUTPUT 3| O N|5 l4 NEGATIVE VOLTAGE SUP PLY INVENTOR GEORGE H. NIBBE ATTORNEY United States Patent SAWTOOTH GENERATOR George H. Nibbe, Chicago, 11]., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application March 26, 1946, Serial No. 657,181

3 Claims. (Cl. 250- 36) This invention relates to electronic time base generators, and more particularly to time base generators having a short flyback time.

Heretofore linear time base generators, or sweep voltage generators, as generally known in the art, have had the undesirable feature of requiring a recovery time 'between successive sweeps that was appreciably long. When circuits having this feature were used as time base generators for generating sweep voltages in oscilloscope applications, it was found that waveforms having a low frequency recurrence rate were capable of clear presentation. However, when the frequency of the observed waveform became higher, the recovery or flyback time of the sweep circuit became a noticeable portion of the total sweep cycle, resulting in a visible return trace on the screen of the cathode ray tube. By the inclusion of additional circuit elements in the oscilloscope circuit, it was possible to blank the return trace from the screen of the cathode ray tube. Regardless of Whether the return trace was blanked or not, in order .to get a true picture of the waveform under study as a function of time without clipping olf the end of the wave, it was necessary that the frequency of the linear time base generator be dilferent than the frequency of the waveform observed which results in difficulties in synchronizing the sweep voltage and the observed waveform.

In this invention the fiyback time of the linear time base generator is very short which tends to overcome the previously mentioned difficulties. Although the problem has been set forth as applying to cathode ray Oscilloscopes, it is not to be inferred that this invention is in any way limited to such an application, but that such was used for illustration purposes only.

A primary object of this invention is to generally improve electronic 'linear'time base generators.

Another object of this invention is to provide alinear time base generator having a very fast recovery time.

A further object is to adapt blocking oscillators for use in linear time base generators having a very rapid recovery time.

These and other objects of the invention will be apparent from the following description when taken with the accompanying drawing which is a schematic diagram of one embodiment thereof.

Referring to the drawing, the cathode of triode electron tube 11 is grounded and the plate connects to the plate voltage supply at terminal 12 through the primary winding of transformer 13. One end of the secondary winding of transformer 13 is connected to ground and the other end connects to one side of condenser 14. The other side of condenser 14 is connected to the grid of triode l1 and also through resistor 15 to the cathode of diode electron tube 20, the plate of which is connected to the plate voltage supply at terminal 12. The grid of triode 11 also connects to the cathode of diode electron tube 21. The plate of diode 21 is connected to ground through resistor 22, and also to the grid of triode electron tube 23. The plate of triode 23 connects directly to the plate voltage supply at terminal 12, and the cathode connects through resistor 24 to a negative voltage supply at terminal 25. The cathode of triode 23 also connects through condenser 30 to the cathode of diode 20. The output is obtained at terminals 31; one of which is connected to the cathode of triode 23 and the other, to ground.

In the operation of this embodiment of .the invention, triode 11 is alternately conducting and non-conducting; the conducting time being an extremely short portion of each period of oscillation. Condenser 14 charges through the grid to cathode circuit of triode 11 during the conducting portion and discharges through resistor 15 during the non-conducting portion of each cycle, as in the normal blocking oscillator circuit. As triode 11 changes from a non-conducting to a conducting state, the change in current in the primary of transformer 13 induces a voltage in the secondary that raises the grid voltage and further increases the plate current. This regenerative actionresults in the grid of triode '11 being rapidly driven into a positive saturation region where further increases in grid voltage have little or no eifec't :upon the plate current. When the grid of triode 11 becomes positive, current flows through the grid to cathode circuit of triode 11 charging condenser 14. Since the grid 'to cathode .resistance is small the time constant for charging condenser .14 is very short. During this portion of the :cycle, the cathode of diode 21 is driven positive isolating the grid of triode 23 and permitting it to rise to the ground potential which in turn raises the output voltage at terminals 31 to its maximum value.

After triode .llhas reached the saturation state, the plate current of triode 11 becomes constant and hence the induced secondary voltage of transformer 13 decreases causing .the grid voltage of'triode 11 to decrease. This causes the plate current of triode 11 to decrease, lowering still further the grid voltage. This regenerative action causes triode 11to 'be returned to the nonconducting state very rapidly, and the grid voltage of triode 1:1 is driven negative .due to the charge which has accumulated on condenser 14. During this portion of the cycle, diode 21 becomes conductive, coupling the voltage drop at the grid of triode 11 to the grid of Hiode 23 and causing the output voltage at terminal 31 to drop sharply.

With the grid voltage :of triode .llbelow ground potential, condenser14 discharges through a long time constant circuit including resistor 15 causing the grid voltage of triode 11 to rise slowly. .As diode 21 is in a conducting state, the rise in voltage of the grid .of triode 11 is coupled to the grid of triode 23 causing the cathode voltage of triode .23 to rise by very nearly the same amount. This rise in voltage on the cathode of triode 23 is coupled by condenser 30, to the junction of resistor 15 and the cathode of diode 20. Thus, as the discharge of condenser :14 tends to raise -the voltage at 'one end of resistor 15, the same rise is coupled to the other end of resistor 15; tending to maintain a constant voltage across resistor 15. A constant voltage across resistor 15 results in a constant charging current for condenser 14 and hence a linear rise in voltage on the grid of triode 11. This linear rise in voltage is coupled to the grid of triode 23, causing a linear voltage rise at the cathode of triode 23 and at output terminals 31.

This rise in voltage continues until the grid voltage of triode 11 reaches the cut-off voltage and the above described cycle is repeated, causing a sawtooth waveform to appear at output terminals 31. The linearity of the output voltage rise depends upon the gain of the cathode follower circuit associated with triode 23. The more nearly the gain approaches unity the more nearly linear is the output voltage rise. The cathode of triode 23 is, therefore, returned to a negative voltage supply at terminal 25 rather than to ground to insure more linear operation of triode 23, and prevent cutting-off triode 23 at any time during the cycle. The time between sweeps for this circuit is the length of time that triode 11 is conducting, which may be reduced to a fraction of a microsecond.

Diode serves to clamp the base of its cathode voltage waveform at the value of the plate voltage supply at terminal 12. As the voltage at the grid of triode 11 tends to rise, the voltage of the cathode of diode 20 tends to rise due to the feed back of the coupling loop comprising diode 21, triode 23 and condenser 30. As the cathode voltage of diode 20 rises the tube becomes non-conducting and the voltage is maintained by the charge on condenser 30. For this reason condenser 30 must be large compared with condenser 14 so that the voltage across condenser 30 does not change greatly during the time that condenser 14 is charging. It will be apparent to those skilled in the art that the frequency of the sawtooth output at terminals 31 is determined by the time constant of resistor 15 and condenser 14, and the frequency may be changed by making these circuit elements variable. The circuit may be synchronized with an external source of voltage applied to the grid or plate circuit of triode 11, or to both by injection in the cathode circuit.

The invention described in the foregoing specification need not be limited to the details shown, which are considered to be illustrative of one form the invention may take. The scope of the invention is defined by the appended claims.

What is claimed is:

1. Electronic apparatus for generating recurring linear sweep voltages having an extremely short flyback time comprising, a voltage source, a first capacitor, a first electron tube having at least a cathode, an anode, and a control grid, said first electron tube being connected as a blocking oscillator, said first capacitor being connected in the control grid circuit of said first electron tube, said first capacitor being charged periodically through said electron tube during the portion of the cycle of said blocking oscillator during which said electron tube is conducting, the quick action of the blocking oscillator providing the short fiyback time of said sweep voltages, a second electron tube having at least a cathode, an anode, and a control grid, said second electron tube being connected as a cathode follower and adapted to have a linear sweep voltage output obtained from its cathode circuit, a first diode electron tube having its cathode connected to said first capacitor and its anode connected to the grid of said second electron tube, said first diode electron tube thereby preventing large positive pulses occurring at said first capacitor from appearing in said output voltage, a second diode electron tube, a resistor, said second diode having its anode connected to said voltage source and its cathode connected to one end of said resistor, the other end of said resistor being joined to said first capacitor, a second capacitor connected between the cathode of said second tube and the end of said resistor remote from said first capacitor, said second capacitor being adapted to provide a feedback path from the output of said cathode follower whereby the voltage drop across said resistor is maintained substantially constant during discharge periods of said first capacitor.

2. A linear sweep voltage generator having a short flyback time comprising a potential source, a first electron tube connected as a blocking oscillator energized from said source, a storage capacitor connected to said blocking oscillator and charged periodically through said electron tube during the portion of the cycle of said blocking oscillator during which said tube is conducting, a first diode and a resistor serially connected between said capacitor and said source to discharge said capacitor toward the potential of said source, said first diode having its anode connected to said source, a second electron tube connected as a cathode follower, a second diode coupling the potential of said capacitor to the input of said cathode follower, said second diode being polarized to isolate said cathode follower for positive potential excursions of said storage capacitor, and means to couple the output of said cathode follower to the junction of said resistor and said first diode, thereby maintaining a substantially constant voltage drop across said resistor during periods of discharge of said capacitor therethrough.

3. A linear sweep voltage generator having a short fiyback time comprising a potential source, a first electron tube having at least a cathode, an anode and a control grid and connected as a blocking oscillator energized from said source, a storage capacitor connected in the grid circuit of said blocking oscillator and charged thereby to the grid potential of said tube, a first diode electron tube and a resistor serially connected between said capacitor and said source to discharge said capacitor toward the potential of said source, said first diode having its anode connected to said source and its cathode connected to said resistor, a second electron tube having at least a cathode, an anode and a control grid and connected as a cathode follower, a second diode electron tube coupling the potential of said capacitor to said control grid of said second electron tube, said second diode being polarized to isolate said cathode follower for positive potential excursions of said control grid of said first electron tube, and means to couple the cathode of said second electron tube to the junction of said resistor and said first diode, thereby maintaining a substantially constant voltage drop across said resistor during periods of discharge of said capacitor therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,425 Geiger et al. Apr. 8, 1941 2,300,451 Smith Nov. 3, 1942 2,396,439 Schlesinger Mar. 12, 1946 2,455,283 Valley Nov. 30, 1948 FOREIGN PATENTS 235,254 Great Britain June 11, 1925 

